JPH04230994A - Improved graphite nipple electrode - Google Patents

Abstract

PURPOSE: To provide a graphite nipple suitable for connection of a graphite electrode by containing mesophase pitch carbon fibers in a nipple extrusion blend, extruding the blend, and baking it at the specified temperature for graphitizing. CONSTITUTION: Carbon fibers manufactured from mesophase pitch having a melting point of less than 350 deg.C are mixed with cokes and a binder, a mixture is extruded to firm an extruded blend. The extruded blend is extruded to form a carbon nipple. The carbon nipple is baked at a temperature exceeding about 538 deg.C for sufficient time, then the baked carbon nipple is heated at a temperature exceeding about 2704 deg.C for graphitization. A graphite nipple suitable for connection of a graphite electrode is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a graphite nipple suitable for connecting graphite electrodes.

0002

BACKGROUND OF THE INVENTION The use of carbon and graphite electrodes in electrothermal processes is well established. For example, ultra-high power graphite electrodes are particularly useful in the production of special alloy steels. The high temperatures necessary to melt metals and other raw materials used in electric furnaces are obtained by passing a high amperage current as an arc is achieved between the electrode and the charge material. The resulting hot arc produces the desired melting, melting, or other electrothermal effect.

[0003] In normal furnace operation, a series of electrodes are attached to each other to form a robust electrode column. As electrodes are consumed during the process, the electrode column is maintained in length by attaching additional electrodes.

The most commonly employed method for connecting electrodes into columns for electric furnaces is through threaded male graphite sections (nipples) connecting the female ends (sockets) of adjacent electrodes. The purpose of this is to construct an assembly. This assembly is called an electrode joint, and there are many arguments in favor of it being the best system for this purpose, considering all factors regarding its operation. It is necessary that the graphite nipple has a reduced cross section compared to the column it supports. Therefore, the nipple is
Usually it should have a higher tensile strength than the rest of the column. However, certain physical properties that contribute to increased strength also accentuate other properties, such as thermal expansion. When the joint is heated, when the radial thermal expansion of the nipple exceeds the thermal expansion of the electrode socket to which it connects (and in which the nipple is enclosed),
And also when the heating joint cools down rapidly, severe tensile (or hoop) stresses are generated in the socket wall by the expanding nipple. These stresses can well exceed the tensile strength of the socket material. As a result of this stressed condition, the socket wall often splits and the column portion beneath the split is often lost.

It is known in the art that it is desirable to use needle (premium) coke in the production of large diameter (eg, 18 inches to 28 inches in diameter), ultrahigh power graphite electrodes. There have been many problems in providing a satisfactory nipple for such electrodes. When premium coke is used for the production of such nipples, it is desirable that the resulting product differs in certain properties from the connected electrode. One such property is the coefficient of thermal expansion (CTE). Electric Furnace Proceedings (1)
976), p. 280, "The CTE of the electrode plus pin (nipple) combination is important for joint connections. Ideally, the best combination is that the pin has a higher radial ) CTE and the electrodes have a higher longitudinal CTE. Such a combination results in a tight connection as the electrodes heat up. On the other hand, the pins have a higher longitudinal CTE and If the electrode has a higher radial CTE, it will result in loose joints and failure.

US Pat. No. 2,969,25 to Thomas
No. 1 relates to a nipple-like carbon electrode joint made by mixing a carbon material such as calcined premium coke, graphite, with a pitch or bituminous material acting as a binder. This patent includes:
Various values of CTE are shown.

US Pat. No. 3,540,76 to Paus et al.
No. 7 discloses an electrode joint made of expanded graphite material located between mating surfaces of the electrodes. The spacer consists of a graphite binder and other materials such as metals, powders or filaments, fiber-based reinforcing materials such as glass fibers, clay, etc. to strengthen the product and improve electrical conductivity.

US Pat. No. 4,352,768 discloses a fiber-reinforced cathode for an electrochemical cell in which the carbon cathode contains graphite, a powder, a carbon black binder, and a carbon fiber additive to strengthen the cathode. Concerning what is included.

It is an object of the present invention to produce an improved graphite nipple suitable for connecting graphite electrodes.

0010

SUMMARY OF THE INVENTION In accordance with the present invention, improved graphite nipples are produced from typical nipple extrusion blends containing carbon fibers. The present invention
It also encompasses the manufacture of such graphite nipples and premium coke electrode columns to which electrodes are connected with such nipples. Although carbon fibers from any source such as polyacrylonitrile can be used, in preferred embodiments carbon fibers prepared from pitch-based materials are used. In the most preferred embodiment, the carbon fibers are prepared from mesophase pitch and the fibers have excellent CTE.

The present invention will be explained in more detail below.

[0012] Graphite electrodes used in electric arc furnaces for the production of steel are usually made from acicular or premium grade coke. The quality of coke, especially premium coke, is often measured by its coefficient of thermal expansion, which value is preferably 9 x 10-7/°C, most preferably 3 x
Should not exceed 10-7/°C. However, the electrodes are prepared from coke having a particle size distribution from a maximum particle size of about 1/2 inch to a fine powder on the order of microns in size. In a preferred but non-critical embodiment, the coke particle size is between 10 and 5
0% by weight is +20 mesh and at least 2 of the particles
0% by weight would be less than 40 mesh. The particle size distribution and structure of the petroleum coke feedstock in the electrode is substantially retained throughout the electrode manufacturing process. The resulting graphitized sample can be examined by microscopic methods so that the final graphite product can be characterized in part by the particle size distribution and structure of the raw material.

In carrying out the electrode and/or graphite nipple manufacturing process, calcined shredded petroleum coke is mixed with a binder, typically coal tar pitch, and a small amount of iron oxide. Iron oxides are used to control the "puffing" of high sulfur petroleum coke during the subsequent electrode graphitization process.A small amount of viscous petroleum can also be added to this mixture as a lubricant. This mixture of coke, pitch and iron oxide is extruded at a temperature close to the softening point of the pitch to form a green electrode or carbon nipple having approximately the required final dimensions. The length can vary from about 18 to about 28 inches, and nipples fabricated from graphitized products typically have a diameter of about 8 to 12 inches.

The green electrode has a temperature of approximately 1000 degrees Fahrenheit (53 degrees Fahrenheit).
8°C) to about 2000°F (1093°C), preferably from about 1400°F (760°C) to about 1°F
It is fired at a temperature of 800 degrees (982 degrees Celsius), during which the binder is carbonized to form a solid body. Generally, residence times of about 2 weeks to about 5 weeks are employed in the firing process. Following this firing process, the electrode may be impregnated with an impregnating pitch and refired to ultimately provide an electrode or carbon nipple with higher density and strength and lower electrical resistance.

Coal tar pitch is the preferred binder, but petroleum pitch obtained as a by-product in the naphtha cracking industry, natural asphalt and heavy oil, high carbon content pitch obtained from petroleum asphalt, as well as petroleum pitch obtained as a by-product in the naphtha cracking industry, as well as high carbon content pitch obtained from petroleum asphalt, are used in various industrial production processes. Other pitches may be used, such as other materials having the properties of pitch obtained as a by-product. Additionally, mesophase pitch can also be used. "Mesophase" is synonymous with liquid crystal, and is a technical term indicating a state of matter intermediate between a crystalline solid and an isotropic liquid.

The final process step is graphitization. The fired carbon electrode and/or carbon nipple article is placed in a furnace surrounded by insulating material and heated to 5° F. for a sufficient time to graphitize the electrode or carbon nipple.
Temperatures up to 400 degrees Fahrenheit (2982 degrees Celsius), particularly from about 4900 degrees Fahrenheit (2704 degrees Celsius) to about 5400 degrees Fahrenheit (2704 degrees Celsius)
Heat to a temperature of 982°C). These temperatures are necessary to convert the amorphous carbon in the electrode and nipple articles to the crystalline graphite state. Generally, a residence time of about 5 days to about 14 days is sufficient to convert the carbon electrode to the graphite state.

The graphite article from which the nipple of the present invention is produced is processed similarly to the electrode, up to the step of mixing the shredded premium (calcined) coke with a carbonaceous binder and iron oxide to form an extrusion blend. Compared to the electrodes mentioned above, the major difference in preparing nipple products is the addition of carbon fibers prepared from mesophase pitch to the extrusion blend before extrusion. The composition of the raw materials used in the preparation of nipple products is usually about 40 to about 72% by weight, preferably about 45 to about 65% by weight, premium coke, about 5 to about 2% by weight.
5% by weight, preferably about 8 to about 20% by weight,
Carbon fiber prepared from mesophase pitch, about 20 to about 30% by weight, preferably about 22 to about 28%
% by weight of a binder, and from about 0 to about 3% by weight, particularly from about 0.01 to about 2.5% by weight, of iron oxide. When present, the optional petroleum-based lubricant may constitute up to about 5%, preferably from about 0.01 to about 4% by weight of the feedstock.

After graphitization, the product is processed into the desired nipple product. It is usually tapered and threaded from the middle to each end. To accommodate the nipple, the electrode is drilled and shaped at each end and threaded to receive the nipple.

The carbon fiber used in the extrusion blend of the nipple article of the present invention is made from pitch. Suitable feedstocks for the preparation of mesophase pitch, which is a precursor of carbon fibers in the present invention, include heavy aromatic petroleum streams, ethylene cracker tar, coal derivatives, petroleum tar,
It contains materials such as fluidized catalytic cracker residue and aromatic fractions and has a boiling point of 650 degrees Fahrenheit to 950 degrees Fahrenheit. Mesophase pitch is conveniently produced by heat soaking a carbonaceous feedstock at a temperature of about 350°C to about 500°C for about 1 hour to about 30 hours. Typically, the heat soaking step is performed using an inert atmosphere, such as nitrogen gas. Air, oxygen or other oxidizing gases may be blown into the carbonaceous feedstock to increase production of mesophase pitch. During this heat soaking step, a portion of the carbonaceous feedstock, eg, mesogens, combines to produce mesophase pitch separated within the carbonaceous feedstock. The mesophase pitch thus produced contains molecules with an aromatic structure that associate with each other by interaction to form optically ordered liquid crystals, which are arranged in a special manner during the heat soaking process. , which can be observed using an optical microscope using polarized light.

The mesophase pitch thus produced has a softening point of less than 350°C. The softening point of mesophase pitch is determined by the Mettler method, which is widely accepted as a standard for evaluating precursor pitches.
Determined by softening point techniques or by thermal stage microscopy. When utilizing the thermal stage microscopy method, the mesophase pitch is heated under polarized light in an inert atmosphere on a microscope thermal stage. The temperature of the mesophase pitch is increased at a controlled rate until it begins to deform. The temperature at which the mesophase pitch begins to deform is recorded as the softening temperature. The mesophase pitch is preferably about 35
It has a softening point below 0°C. This is because above this temperature, mesophase pitch cannot be spun into carbon fibers that have the desired physical and chemical properties to enhance the properties of graphite nipples.

[0021] In the carbon fiber manufacturing process, mesophase pitch is spun into fiber filaments, oxidatively stabilized at elevated temperatures, such as 350°C, and then heat treated at temperatures of 1800°C or higher in an inert atmosphere. Carbon fibers useful in the present invention vary in diameter and length. Typically, the fiber diameter is about 3 to about 20 microns. The fibers may be cut into convenient lengths of 1/2 inch to 2 inches or more before use. The fibers are elongated with the coke and are random in length in the coke as a result of the action of the extender on the fibers. The fibers are substantially oriented by the extender to provide the desired CTE effect described.

In a preferred embodiment, the carbon fibers after graphitization are greater than 200 x 103 psi, preferably from about 200 x 103 psi to about 300 x
Has a tensile strength of 103 psi and 55 x 106 ps
i, preferably from about 55 x 106 to about 120 x 106, and 250 x 10
It has an electrical resistivity of less than 6 ohm centimeters.

Carbon fibers made from mesophase pitch have zero or negative longitudinal CTE values and are preferred. The use of such carbon fibers results in a reduced longitudinal CTE than that obtained from premium coke alone;
A nipple with increased cross-sectional CTE is obtained over that provided by premium coke. As used herein, "mesophase carbon fiber" refers to carbon fibers made from mesophase-containing pitch that have zero or negative machine direction CTE.

Mesophase pitch is melt spun to form fibers, oxidized and crosslinked, and thus remelted and carbonized. As-spun fibers are amorphous but highly oriented, with molecular layer planes aligned along the fiber axis. The full width at the widest half of the distribution of aromatic molecule orientation along the fiber axis is about 25 degrees to 30 degrees, with about 3/4 of the lamellar planes within an angle of ±15 degrees of the fiber axis. It should be noted that cellulosic carbon fibers such as rayon and polyacrylonitrile must be hot drawn at temperatures above about 3500° C. to obtain very dissimilar patterns.

Because the as-spun carbon fibers have a highly oriented and systematic structure, they can be thermally graphitized and have extremely high moduli (eg, 1 ×106 MP
A) can be achieved.

In the process of making graphite nipples, the addition of mesophase carbon fibers reduces the longitudinal CTE and increases the transverse CTE of the nipple compared to that obtained using premium coke alone. The exact rate of machine direction CTE reduction and cross direction CTE increase will depend on the nature of the premium coke and the as-spun carbon fiber properties used.

The techniques and conditions employed in the production of carbon fibers are known and therefore they do not constitute an important feature of the invention. Carbon fibers from pitch, whether petroleum or coal-based, or carbon fibers from polyacrylonitrile carbonization processes can be used. However, fibers from pitches containing mesophase pitch are preferred for economics and properties. A general disclosure of methods for making carbon fiber is contained in US Pat. No. 4,005,183 issued to Singer.

[0028]

EXAMPLES The following examples explain the present invention more specifically. This example is intended to illustrate the invention, but not to limit it.

Example The premium coke used in this example was a powder sample.
3/4 inch diameter electrode longitudinal CTE 1.5 x 10-7/℃
It was a commercially available coke with a 70 green electrodes
Extruded from a millimeter diameter extruder, fired,
Graphitized. The coke composition of the base case and that of the carbon fiber case is one containing 25 wt% 8x16 mesh particles, 25 wt% 20x40 mesh particles, and 50 wt% powder (-100 mesh particles). Ta. This powder further contains 50% by weight of 100×
It was divided into 200 mesh particles and 50% by weight of -200 mesh particles. The binder pitch was coal tar pitch (N
o. 30 medium). The mesophase pitch used in this example was commercially available spool fiber (P55) supplied by Union Carbide. This union carbide fiber is described in Fuel, Volume 60, Number 9, Pages 839-847, September 1981. This P55 mesophase pitch is -0.
It had a CTE of 9 x 10-6/°C and a diameter of 8 to 12 microns. The fibers were cut into 2 inch lengths and included in the extrusion blend. The composition and properties of the extruded blends with and without mesophase carbon fibers are shown in the table below.

Table
Characteristics of electrode nipple
Apparent Electric Breakdown Blend CTEL CTET Density Resistivity Coefficient Composition
x10-7/℃ x10-7/
°C g/cc x10-4in-o
hm psi basic case 4.3
20.6 1.62 3.2513
40 76% premium coke 22% coal tar pitch 1.5% iron oxide 0.5% lubricant mesophase carbon fiber addition 1.3 21.4
1.60 3.571156 63% premium coke 22% coal tar pitch 1.5% iron oxide 0.5% lubricant 13% carbon fiber 13% by weight mesophase pitch added as shown in the table above Then, the vertical CTE is 4.3×10
-7/°C to 1.3×10-7/°C, lateral C
TE is 20.6×10-7/℃ to 21.4×10-
7/°C and also maintained the density electrical resistivity and strength levels of the base case. The appearance and texture of the base case and mesophase carbon fiber-based electrodes were substantially the same.

Claims (12)

[Claims] 1. (a) adding carbon fibers made from mesophase pitch having a melting point below 350° C. to calcined coke and a binder to form an extrusion blend; (b) extruding the extrusion blend; forming an elongated carbon nipple; (c) subjecting the carbon nipple to a temperature greater than about 538° C. for a sufficient time to cause the carbon nipple to bake;
and (d) subjecting the calcined carbon nipple to a temperature above about 2704° C. for a sufficient time to graphitize the carbon nipple. 2. The method of claim 1, wherein the mesophase pitch is spun into carbon fibers. 3. The method of claim 1 further comprising adding a small amount of iron oxide to the extrusion blend prior to extrusion. 4. The method of claim 1 further comprising adding a small amount of lubricant to the extrusion blend prior to extrusion. 5. The method of claim 1, wherein the graphitized carbon fibers of the graphite nipple have a tensile strength greater than 200 x 103 psi. 6. The method of claim 1, wherein the graphitized carbon fibers of the graphite nipple have an elastic Young's modulus greater than 55×10 6 psi. 7. (a) about 8% to about 20% by weight of carbon fiber made from spun sophase pitch and about 45% to about 65% by weight of premium coke;
and about 22% to about 28% by weight of a binder, forming an extruded blend; (b) extruding the extruded blend to form an elongated carbon nipple; (c) forming the carbon nipple into an elongated carbon nipple; 760℃ to about 982℃
(d) subjecting the fired carbon nipple to a temperature in excess of about 2,704° C. for about 5 days to about 14 days; A method for producing a graphite nipple suitable for connecting graphite electrodes, comprising the step of providing a graphite nipple to produce a graphite nipple. 8. The method of claim 7, wherein the mesophase pitch used to produce the carbon fibers has a melting point of less than 350°C. 9. The method of claim 7 further comprising adding from about 0.01% to about 2.5% by weight of iron oxide to the extrusion blend. 10. About 0.01% by weight in the extrusion blend.
8. The method of claim 7 further comprising adding from about 4% by weight of a lubricant. 11. The graphitized carbon fiber of the graphite nipple has a pressure of about 200 x 103 psi to about 3
8. The method of claim 7 having a tensile strength of 00 x 103 psi. 12. The graphite nipple is approximately 55×
8. The method of claim 7, having an elastic Young's modulus of 106 psi to about 120 x 106 psi.